1. Field of the Invention
This invention relates to quench hardening of steel casings.
2. Prior Art
Steel oil well casings subjected to hydrogen sulfide environments must be fully martensitic in structure to resist corrosion and cracking.
Quench hardening, i.e., austenitizing by heating the steel casing to above the transformation range and then cooling rapidly enough to transform the austenite to martensite by water spray impingement has been used, particularly on the external surface, but also in combination with internal cooling. See, e.g., U.S. Pat. No. 3,140,964 to the hardening of a metal pipe with an external quench, and U.S. Pat. No. 3,294,599 to the treating of low carbon steel tubular members by continuous internal and external quenching.
Where only external quenching is used, quench cracks may result if the quench spray enters the open ends of the tubular member before it is cooled. One approach to solving this problem has been to apply an end plate that blocks entry of quench spray. Another has been to move the spray unit with the pipe or to stop the pipe as the trailing end approaches the spray head, until it is sufficiently cooled. All of these approaches have inherent inefficiencies.
Where internal and external quenching is required, as for casings that have relatively thick walls, support of the internal quench and removal of the casing or quenches after quenching is completed has been a problem that hampers efficient work flow and handling.
Such known approaches are not satisfactory where large quantities of casings must be processed efficiently and economically. Thus, a definite need has existed for an improved method and apparatus for accomplishing the quench hardening of steel casings in a manner that achieves a fully martensitic structure, and that permits a continuous product flow during the processing yet without the need for multiple, alternately used, structures.